Leaving for London this week. 1) should I get pounds before I leave
to have cash for transportation from airport to city
No. You will be robbed blindly by your local bank if you do that. You can get cash in Central London's cash machines after you take public transport into the city. I also recommend you avoid Travelex at the airport -- highway robbery. Find out from your bank if it has reciprocity with any British banks so you aren't charged a fee for you using an ATM. For example, I only pull cash from Barclays cash machines as I am a Bank of America client. If you have credit union ATM card, there already may be no fees in place when you take out cash at a UK bank's cash machine. Find out today as you are leaving soon.
If you plan on returning to the UK within a 5 years or so, take out £s from the cash machine to bring home. The exchange rate is fantastic for you and the £ WILL REBOUND once things calm down with the UK leaving the EU. I'm planning to do just that on my trip in March.
2) should I use credit card for every purchase? I believe my bank
charges every time I do, or mayb it’s only when I use ATM?
It depends on your credit card. I have a Travel Rewards Visa card that does NOT charge international fees. If you don't have a credit card like that, look at all of your credit cards and find out which has the cheapest int'l fees and then use that exclusively. Hopefully that one will also have some kind of points program.
As Nigel mentioned, make whichever card you choose with the cheapest int'l fees as your Apple Pay/Android Pay/Samsung Pay card. Contactless is great.
3) most places take cards, correct? Better to use as CC or debit?
I never use debit to pay for any product or service; the money comes out of your account immediately. If there is a problem, you have to waste time with your bank trying to get your money back. As for "most places" taking plastic? Perhaps a few small shops may not but London is pretty easy.
When I was in Ireland last year when I’d purchase something it always
came up if I wanted to use dollars or euros.
Always pick the local currency. NEVER choose dollars as you will be ripped off royally.